Physical and biological processes in the North Atlantic during the first GARP Global Experiment

Abstract

The seasonal cycle of the phytoplankton biomass in the North Atlantic Ocean (20°S to 70°N) is discussed using monthly mean surface pigment concentration fields from the Nimbus 7 coastal zone color scanner (CZCS) for the period of December 1978 through December 1979. The temporal and spatial distributions of surface pigments are compared with quantities derived from the First GARP Global Experiment (FGGE) wind data set (1000‐mbar streamlines, Ekman upwelling, horizontal Ekman transport and |τ|3/2), Climate Analysis Center (CAC) sea surface temperature (SST) fields, concurrent hydrographic station data, and climatological mixed‐layer depths and heat fluxes. The objective of the analysis is to clarify the role of some physical mechanisms that regulate the supply of nutrients to the euphotic zone at different locations in the basin and at different times of the year. It is observed that during this period, the spring bloom was, in most locations, sustained for 2 to 3 months. At a given latitude the onset of the spring bloom varied by several months and, in some cases, could be referred to as a fall bloom. In other cases there were both spring and fall blooms. Time series at 11 selected sites throughout the basin are used to illustrate, in detail, the relation of phytoplankton biomass to local physical forcing. Finally, separate analyses of the mean pigment concentrations for the shelf and open ocean regions indicate that the phytoplankton of both regimes had spring and fall blooms of equal magnitude, the spring bloom being located at northern latitudes and the fall bloom in the equatorial region. Analysis of the open ocean Ekman upwelling field shows similar peaks approximately 2 months in advance of the surface pigment concentration peaks.